The invention relates to a screen printing machine having a printing table and a squeegee which can be moved over this printing table and is held at a support by means of a squeegee holder, which support moves in guide rails, via guiding devices, wherein this squeegee can be adjusted at a distance as well as at an angle with respect to the printing table and can be pressed against the printing table by means of pressure cylinders.
Screen printing machines are known (Brochure Model 3000 of Thieme Maschinenfabrik, Printed 3/86), in which the squeegee with its squeegee holder is arranged fixedly at the support which can be moved reciprocatingly by means of claw-type clamping pieces. In these constructions, the pressure of the squeegee is carried out by a vertical adjustment of the support, which takes place in that the position of the support is adjusted manually by means of lateral threaded spindles with a precision scale. The lift-off movement of the squeegee, when the supports are moved back, takes place mechanically by means of corresponding eccentrics. In the case of these constructions, the printing quality depends on the exact adjustment of the support with the squeegee. When the lengths of the support are large, and when the pressures of the squeegee are high, the deformation occurring in the support may result in errors during the printing operation.
Therefore, screen printing machines have also become known (Brochure Thieme Maschinenfabrik Model 2000 E), in which in order to achieve a uniform contact pressure onto the squeegee, the support is equipped with an additional squeegee support which is disposed with respect to the support, so that it can be moved by a small amount. This squeegee support is pressed against the printing table, by means of pneumatic cylinders, during the operation of the squeegee. The lifting-off of the squeegee during the return movement, as in the case of the other known constructions, takes place by the mechanical raising of the whole support. The angular adjustment of the squeegee with respect to the printing table also takes place by a swivelling of the whole support. Even though in the case of these constructions a bending of the support, which takes place at high squeegee pressures, cannot result directly in a deformation at the squeegee, the use of such constructions is possible without any impairment of the printing only if the squeegee is held symmetrically with respect to the center of the support. During the production of printed material which requires only a part of the width of the printing table, for example, in the case of the application of a lateral printing to an otherwise finished poster, the squeegee, which in this case has a much shorter construction than the support, has to be mounted off-center at one side of the support. However, the weight of the squeegee support, which extends over almost the whole width of the support and can be moved with respect to the support, will then have a negative effect on the printing quality because it has a one-sided effect on the squeegee. The consequences are the more disadvantageous, the lower the contact pressure to be used for the printing. In addition, the known constructions are relatively heavy and therefore expensive because of the necessity of the arrangement of two supports.
An object of the invention is therefore to construct a screen printing machine of this type such that the position of the squeegee at the support and the size of the squeegee have no influence on the printing quality.
In order to achieve this object, it is provided according to the invention in the case of a screen printing machine of the initially mentioned type that the support, is disposed directly in the guide rails, with both ends, and that the pressure cylinders are double-action pressure cylinders and are designed as guiding devices which are longitudinally slidably fastened at the support and are equipped with swivel guides for the squeegee holder. By means of this construction, on the one hand, the vertical-adjustment devices for the support are not required at its ends. On the other hand, the devices for the raising of the support during the return movement are also not necessary, and it is no longer necessary to carry out the angular adjustment of the squeegee by means of a swivelling of the whole support. As a result of the construction according to the invention, it is sufficient to arrange the pressure cylinders, which are constructed as guiding devices, directly at the support. Since these guiding cylinders can be acted upon by the same pressure, the squeegee holder with the squeegee rubber, which now is fastened directly at the guiding cylinders, can be pressed uniformly against the printing table without the damaging effect of deformations of the support which occur particularly at high pressures and with long squeegees. In the new construction, the squeegee holder itself is also not bent. By mean of the arrangement of the swivel guides at the cylinders, the angular adjustment of the squeegee may also take place in a very simple manner, without the requirement of disposing the support in its guide rails in a costly manner. The invention therefore permits a particularly simple construction of the squeegee device, which also has the advantage that the lift-off movement of the squeegee can be carried out by the guiding cylinders. In this manner, a steady straight-line motion of the support can also be provided which is not affected by adjusting devices between the rails and the support. The guiding can therefore become very precise. The printing quality may be improved, since also any possible bending of the support has no effect. Finally, it makes no difference whether or not, in the new construction, the squeegee is arranged symmetrically with respect to the support. By means of the complete uncoupling of the squeegee suspension from the support, deformations of the support remain without any influence on the contact pressure of the squeegee. As a result of the new construction, the whole arrangement can also be much lighter and the manufacturing expenditures become lower.
In advantageous preferred embodiments of the invention the support may be constructed as a hollow-profile support, for example, of light metal, and will then be very sturdy, but still light. This hollow-profile support may be provided with grooves at at least one outer side, in which the pressure cylinders, which are housed in the holding devices, may be clamped in a longitudinally slidable manner.
Certain preferred embodiments advantageously and in a simple manner provide that the swivel guide is comprised of a circular link guide, which is mounted at the lower end of the piston rod of the pressure cylinders and which, at the counterpart which is slidable in the link, is provided with a clamping profile piece which corresponds to the profile of the squeegee support to be applied and to the profile of fastening claws which are known per se. In this Case, it becomes possible to transfer the swivel shaft of the link guide in the area of the squeegee edge facing the printing table, so that, when the angular position is changed, the slope of the squeegee rubber is changed with respect to the table, but not the position of the squeegee edge in the direction of the squeegee movement with respect to the printing table. An adjustment of the starting position of the support is therefore not necessary again when the angular position of the squeegee is changed.
Finally, advantageous embodiments are contemplated with not only one squeegee, but also a presqueegee assigned to the support. In this case, the same pressure cylinders may be used also for the guiding of the presqueegee assigned to the support. In order to avoid that the presqueegee rests on the screen with an excessive pressure, when the return movement of the squeegee takes place, the pressure cylinder assigned to the presqueegee may be provided with a vertical-adjustment device, which may simply consist of an adjusting or counternut, which is assigned to the end of a correspondingly constructed piston rod of the pressure cylinder which projects out of the cylinder on top.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.